1. Field of the Invention
The present invention relates to a technology for correcting a servo signal on a servo track generated using a common pattern.
2. Description of the Related Art
Conventionally, an increase of storage capacity of a disk device, such as a magnetic disk device, has been in demand. To increase the storage capacity of the magnetic disk device, it is necessary to reduce the track pitch of the magnetic disk and write a track that includes servo information (servo track) in the right position. One of the methods for writing such a servo track in the right position is a method in which each individual magnetic disk device is provided with a function of writing a servo track in the right position. However, this method increases the price of an individual magnetic disk device.
For this reason, a method in which a servo track is written on a magnetic disk and then the magnetic disk with the servo track written on is mounted on an individual device is being considered. In such a magnetic disk device, it is necessary to perform a positional correction for accurately positioning the head on a target track and a missing portion correction to a missing portion of the servo track. For instance, information for performing control of positioning onto the target track is recorded on the magnetic disk medium and is called a servo signal. A process of recording the servo signal by a magnetic disk device is called a servo track write (STW), and various methods for the STW have been suggested.
Examples of the STW include a method in which a servo signal is generated by using a common pattern (master medium) such as magnetic transferring, a discrete track medium, and a patterned medium (a pattern that serves as a servo master disk) (servo recording system). In these methods, multiple copies of magnetic disks are produced from a common pattern so as to form a servo pattern on each magnetic disk. Thus, the servo pattern can be duplicated at once, and time efficiency in servo recording can be dramatically improved. However, this method has drawbacks, such as a distorted pattern lowering accuracy in positioning onto a target track or causing a missing portion in a servo track. The positioning on the target track therefore must be performed only after making corrections to the track displacement and corrections to the missing portion and the like that are attributed to the distorted pattern and the like.
For instance, a magnetic disk described in Japanese Patent Application Laid-open No. 2004-342316 is configured in such a manner that a control unit that controls the positioning of a magnetic head in accordance with the positional difference based on the servo information performs feed-forward control of the positioning in such a manner as to follow the measured rotational frequency component of the magnetic disk and eliminate higher frequency components than the rotational frequency.
With a method in which a servo pattern is formed using a common pattern, if there is any unfavorable condition such as lowered accuracy or a missing portion in the master disk that serves as the pattern, the servo pattern is completed with all these conditions included in the servo information on the magnetic disk. Thus, it is necessary to make a correction on each individual magnetic disk device to eliminate such unfavorable conditions. In other words, for a magnetic disk device to correct distortion information of a disk (servo signal) attributed to a pattern or the like with the above conventional technology, it is necessary to correct distortion information by measuring the rotational frequency component of the magnetic disk and higher frequency components than the rotational frequency on the magnetic disk device. This causes a problem that it takes a long time to correct the distortion information of a servo pattern on the magnetic disk device, leading to a problem of making the production time of the magnetic disk device longer and lowering the production efficiency. These problems become larger issues as the recording density of a magnetic disk device increases.